Bi-polar triac short detection and safety circuit

ABSTRACT

A safety circuit to disrupt power to a heating element of an appliance to be powered through a solid state switch (Tri-ac Q 1 ), typically a triac, from an AC power source (V 1 ) having a positive half cycle and a negative half cycle delivering power. A low resistance condition is sensed by detecting either the current through or voltage across the solid state switch during the positive half cycle and the negative half cycle of the AC power line, when the solid state AC switch is not actuated. A fault signal is generated to interrupt power to the heating element, preferably by a crowbar circuit opening a fuse, whenever the low resistance condition is detected.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on U.S. provisional application Ser. No. 61/782,714 filed Mar. 14, 2013.

FIELD OF THE INVENTION

The present invention relates to safety circuits for AC operated appliances, for example though not limited to, heating pads and electric blankets, using solid state switching circuits to activate a load to assure the integrity of a solid state switching circuit controlling activation of the load.

BACKGROUND OF THE INVENTION

Heating pads and electric blankets typically use heating elements powered by AC line voltage, where the temperature of the heating elements is controlled, and the safety continuously monitored to protect against over heating. Typically, power to the heating element of such devices is switched on by a solid state switch such as a triac. The integrity of the triac is a key factor in the safety of the product. Should a triac fail in the shorted condition, continuous heating can result in overheating, and the user could be exposed to suffering burns and the chance of a fire can occur. It is thus important to detect a shorted power switch condition and to disconnect the power before allowing an unsafe condition to develop.

Using multiple circuits in a heating element of a heating pad or electric blanket provides better detection of overheating due to a larger portion of the heating wire being affected by a bunch condition. In this way, overheating can often be recognized sooner by the temperature control circuitry.

The Weiss/Lin U.S. Pat. No. 5,420,397 discloses a safety circuit for a PTC heater wire that detects a break in the wire and quickly turns the power off before an arc can cause the highly flammable wire to catch fire. The circuit of this patent uses a triac to switch on and off the power by a time proportion relative to the heat setting. In one embodiment, two triacs are used in series to mitigate the effect of one of the triacs failing by becoming shorted inasmuch as the second triac would disrupt the power. In another embodiment, a second triac is used in a crowbar circuit to open the power fuse if power to the PTC wire is detected during the off state of the power control triac.

Heating pads and electric blankets typically have higher wattage than is needed to stabilize at the desired temperature. The extra power is typically provided in order to quickly bring the surface of the heating pad or electric blanket up to the desired temperature. This is termed the preheat mode that drives the heater wire to a higher temperature for a short period of time. After the preheat mode, a controller measures the temperature and maintains the wire at a target temperature according to the setting selected by the user. In this case, the power to maintain the desired temperature may be as little as 20% of the total available power. The solid state switch, typically a triac, can fail by becoming shorted in either the full wave or half wave condition. Even a failure in the half wave condition could provide 50% of the power continuously and eventually result in overheating of the heating element. Attempts to detect a triac short in the positive half cycle only, such as in the Kohn/Levy patent application US2013/0015174 A1, leave vulnerable the situation where the triac may be shorted in the negative half cycle, and a runaway temperature could result.

Appliances other than heating pads and electric blankets have heating elements powered from an AC line and use triacs to switch the power on and off to control temperature by connecting the heating element to AC power when the temperature is below a preset value, and disconnecting the heating element from the AC power when the required temperature is reached. Other types of AC operated appliances use electronic AC switches to operatively connect and disconnect power to the load. Failure resulting from a shorted electronic power switch (e.g., a triac) in such appliances also can lead to unsafe and uncontrollable temperature raise of the heating element, or other unsafe conditions.

In microcontroller (MCU) based circuits, an MCU is used to measure temperature of the heater wire and provide a control signal for the triac. These MCU circuits are quite often powered from a non-isolated low voltage power supply connected to the power line, and providing just single polarity DC voltage, e.g. +5V. Having a single polarity power supply provides obstacles for direct detection of the opposite polarity or bipolar signals.

In the case of a heating pad or other appliances having a heating element powered from an AC line, it has been found to be advantageous to use two circuits, one circuit is powered by the positive half cycle of the AC power line, typically 120 VAC, and the second circuit is powered by the negative half cycle of the AC power line, as described in United States patent application US20130134149. Heating elements typically used have positive temperature coefficient characteristics, for example when nickel is used, and the temperature is determined by the measured resistance for both circuits. The first circuit resistance is measured during the positive half cycle of the AC power line, and the second circuit resistance is measured during the negative half cycle. The requirement to measure the resistance in the negative half cycle for the second circuit has led to a requirement to determine the conduction of the triac both when power should be applied and when power should not be applied. Conductance of the triac when the power should not be applied is indicative of a triac short. Triacs can be shorted for either the positive or negative half cycles of the AC line, or even for both cycles. For single circuit and multiple circuit heating pads failure to detect the AC switch failure for either half cycle may lead to power during that cycle being applied to the heating element, and cause overheating.

The same principles apply to other types of the electronic AC switches, e.g. MOSFET based AC switches, BJT based AC switches, thyristor based AC switches, triac equivalents, etc.

With an appropriate circuit arrangement, the MCU can detect failures and shut the circuit down under certain abnormal conditions, such as overheating, a wire break or when the triac fails short.

Referring to FIG. 1, illustrating a prior art circuit, MCU U1 triggers the Triac Q1 to energize the heating element HT1 in response to the reading of a temperature sensor (not shown.) The MCU U1 periodically stops the trigger pulses to evaluate the condition of the triac Q1. The Triac Detector circuit 10 comprising a transistor Q2, reverse voltage protection diode D3, current limiting resistor R2 and a load resistor R1, detects voltage presence on the MT2 terminal of the triac Q1, and outputs the low level to the MCU when voltage is present, and the high level when there is no voltage (triac conducting). The MCU compares the response of the triac detector circuit to the current status of the triac Q1 control signal (trigger). If the control signal is present, and the triac is expected to conduct current, the MCU expects a high level form the detector. When there is no control signal, the MCU expects the low level from the detector during the positive half cycle of the power line voltage. Triac short failure is detected when the MCU receives the high level while the triac is not triggered. When such failure is detected, the MCU triggers the Crowbar circuit, comprising a Triac Qc and a current limiting resistor Rlim, which blows the Fuse F1 and disconnects the Heater HT1 from the power line.

The prior art circuit of FIG. 1, however, is not responsive to a negative voltage at MT2 terminal of the triac Q1. The triac, however, has both negative and positive switching facilities (for positive and negative half cycles of the power line) that can fail independently. The circuit of FIG. 1 does not protect against the short failure of the negative half cycle control part of the triac Q1. Therefore, there is a need to provide detection of the triac failure for both the positive and negative half cycles of the power line.

OBJECT OF THE INVENTION

It is, therefore, an object of the present invention to provide for an appliance utilizing a heating element powered from AC supply, a circuitry that detects a short in a solid state switch such as a triac in both the positive and negative half cycle of an AC power supply so that in this way an appliance utilizing a heating element powered by the positive half cycle and the negative half cycle of the AC power will be protected from overheating from a failure of the solid state switching element in either the positive or negative half of the AC cycle.

SUMMARY OF THE INVENTION

According to the invention, a safety circuit is provided for use in disrupting power to a heating element of an appliance to be powered through a solid state switch, typically a triac, from an AC power source having a positive half cycle and a negative half cycle delivering power. A low resistance condition is sensed by detecting either the current through or absence of voltage across the solid state switch during the positive half cycle and the negative half cycle of the AC power line, when the solid state AC switch is not actuated. A fault signal is generated to interrupt power to the heating element, preferably by a crowbar circuit opening a fuse, whenever the low resistance condition is detected.

In one embodiment, the solid state switch is selectively triggered to supply power to the heating element during only a predetermined number of cycles of the applied AC power source, providing a duty cycle limited average to the load. A circuit interrupter in series with the heating element and solid state switch disrupts power to the heating element when a larger current is established indicating the solid state switch is passing more than the predetermined number of cycles to the heating element.

In another embodiment, the safety circuits detecting either the current through or absence of voltage across the solid state switch during the positive half cycle and the negative half cycle of the AC power line, when the solid state AC switch is not actuated, provide a logical fault related signal to an MCU or other logical circuit, which actuates a circuit interrupter, e.g. a crowbar circuit, to disrupt power to the heating element.

In yet another embodiment, the safety circuits are used with an analog input of the MCU, providing simplified means to detect either the current through or absence of voltage across the solid state switch during the positive half cycle and the negative half cycle of the AC power line, when the solid state AC switch is not actuated. When the fault condition is detected, the MCU actuates a circuit interrupter, e.g. a crowbar circuit, to disrupt power to the heating element. This embodiment is further adapted to a heating appliance with dual heating elements each formed of a positive temperature coefficient flexible wire and individually powered through a solid state switch by a respective half cycle of the AC power source, where the provided bi-polar current detection means are further utilized for independent temperature control of each of the positive half cycle operated and the negative half cycle operated heating circuit.

These and other objects, features and advantages of the present invention are provided by a circuitry providing a safety circuit that detects a short in either the positive or negative half cycle of the solid state switching element to initiate a crowbar circuit to increase current to the fuse through which AC power is supplied to the appliance to open the fuse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a prior art circuit for the AC switch failure detection, limited to the positive half cycle;

FIG. 2 is a circuit diagram for the AC switch failure detection for both AC half cycles by sensing a voltage drop across the solid state switching device;

FIG. 3 is a circuit diagram for the AC switch failure detection during both AC half cycles by sensing current through the solid state switching device;

FIG. 4 is a circuit diagram of an alternate circuit for the AC switch failure detection during both AC half cycles;

FIG. 5 is a circuit diagram for an alternate circuit for the AC switch failure detection during both AC half cycles;

FIG. 6 is a circuit diagram for an alternate circuit controlling power to a heating element from an AC power source during both AC cycles;

FIG. 7 is a circuit diagram for an alternate circuit for the AC switch failure detection during both AC half cycles by using analog inputs;

FIG. 8 illustrates the relation of the input analog signals to the logical signals in the MCU of the circuitry of FIG. 7;

FIG. 9 is a circuit diagram for an alternate circuit for the AC switch failure detection during both AC half cycles by using analog inputs;

FIG. 10 illustrates the signals of FIG. 9;

FIGS. 11 a and 11 b illustrate circuit diagrams for an alternate circuit controlling power to a heating element from an AC power source during both AC cycles by analog inputs;

FIG. 12 is a circuit diagram for a preferred circuit for the AC switch failure detection during both AC half cycles in a dual circuit heating pad controller;

FIG. 13 illustrates the signals of the circuitry of FIG. 12;

FIG. 14 is a circuit diagram for a duty cycle enabled protection responsive to the AC switch failure at either half cycle;

FIG. 15 illustrates the signals of the circuitry of FIG. 14;

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The triac detector circuitry 12 shown in FIG. 2 detects a short during either half cycle of AC power controlled by a triac to a heating device. A PNP transistor Q3 provides a required negative voltage detection capability. This bi-polar voltage detector operates as follows. When the triac Q1 is conducting, the voltage drop across the triac is below 1 Vrms. This voltage is too small to turn the switch Q2 on. Voltage divider R3 R5 keeps Vbe of Q3 at about 0.15V, which keeps the switch Q3 off as well. The detector output is high. When positive line voltage is present, Q2 turns on, and the detector output is low. Diode D2 protects the base of Q3 from reverse voltage. When negative line voltage is present, Q3 turns on, and turns on the switch Q2, and the detector output is low.

The detector of FIG. 2 detects triac conduction based on the voltage drop across the triac.

Another method of detection of the triac conduction is based on sensing current passing through the triac. The circuit of FIG. 3 shows such a detector using a current transformer (CT). This circuit outputs positive voltage to the MCU when the triac Q1 is conducting on either the positive or negative half cycle of the AC power line voltage. The voltage is proportional to the magnitude of current passing through the triac. The CT turn ratio is selected so that the nominal current through the triac creates voltage on the output of the detector corresponding to the high logic level. Resistor R3 limits input current of the MCU at current spikes.

An AC optocoupler (OC) enabled circuit of FIG. 4 provides bi-polar detection of the triac Q1 current based on a voltage drop across the current sense resistor R8. The output of the circuit is high when the triac is off, and goes low when any polarity current is detected. The values on the schematic correspond to the heater current of about 0.5 Amp.

A simple and inexpensive bi-polar current detector is presented on FIG. 5. This current detector comprises two NPN switches Q2 and Q3 that share the load resistor R1. The output of the circuit is high, when the triac is off. Positive current through the current sense resistor R6 turns the switch Q2 on, and the output goes low. The output goes low, when negative current through R6 turns Q3 on. Resistors R4 and R5 limit the base currents of the corresponding transistors. R6 value on the schematic corresponds to the heater current of about 0.5 Amp. The current sense resistor of the circuit of FIG. 5 dissipates about the half of the heat of the current sense resistor of FIG. 4.

An inductor or a capacitor may be used in place of current sense resistor of FIGS. 4 and FIG. 5 to reduce heat generated by the current sense component. To further reduce heat, and make the circuit work on a wide range of currents, the circuit of FIG. 6 uses a pair of inversely paralleled diodes D1 and D2 in place of a current sense resistor.

The circuits discussed up to this point are designed to work with digital inputs of an MCU or any logic driven protection circuit. Having an analog input available opens up other opportunities for bi-polar voltage and current detection. An on-board or external A-to-D converter (ADC) or a pair of on-board or external analog comparators may be used as an analog input of the MCU or any other logic driven protection circuit.

With the analog input, the voltage detector circuit of FIG. 2 can be transformed into a simple arrangement of FIG. 7. Voltage divider R1 R3 provides a bias for the analog input of the MCU. Resistor R2 together with the bias network forms a voltage divider that scales down the power voltage. A voltage comparator or an ADC of the MCU detects the status of the triac Q1 as shown on FIG. 8.

FIG. 9 depicts a bi-polar current detector version for an MCU with an analog input. This circuit features a voltage divider R1 R2 that sets the bias at 1 V, and a DC blocking capacitor C3. A voltage comparator or an ADC of the MCU detects the status of the triac Q1 as shown on FIG. 10.

The two circuits shown on FIG. 11 A and FIG. 11 B demonstrate yet another version of the bi-polar current detector working in conjunction with an analog input of the MCU. Both circuits feature a small value current sense resistor and a bias circuit that shifts a zero at the MCU input up by about 500mV. The value of the current sense resistor is selected for a heater current of about 0.5 Amp. The circuits work similar to the detector of FIG. 9.

Bi-polar current detection circuits of FIGS. 11 a and 11 b can be used for temperature control of the heating pad with a heating element using a positive temperature coefficient (PTC) wire that changes its resistance with temperature. The bi-polar detection capability enables independent temperature control of each circuit of the dual circuit heating pad, where one circuit is powered by a positive half cycle of the power line, and another circuit by the negative half cycle. An example of such controller is shown on FIG. 12.

The circuit of FIG. 12 uses an MCU (not shown) that employs an ADC to measure a voltage drop across the current sense resistor R6 during the positive and negative half cycles of the AC power line (V_(PTC) _(—) _(POS) and V_(PTC) _(—) _(NEG)) to a dual circuit heating pad. The MCU also measures the power line voltage scaled down by a voltage divider R9 R10 . To increase resolution, in this example the line voltage V_(LINE) is measured as amplitude of the positive half cycle, taking advantage of the symmetrical nature of the AC power line. The MCU uses the measurements to evaluate heating wire temperature for the positive and negative half cycle operated parts of the heating element independently, as shown of FIG. 13. The resistance of the positive and negative halves of the heating element is measured as ratios of the power line voltage measurement to the amplitude of the corresponding half cycle measurement of the voltage drop across the current sense resistor. The resulting values that directly correspond to the temperatures of the positive and negative halves of the heating element are compared to a preset level, and if the preset level is reached, the heating element part of the corresponding polarity is turned off for the duration of the immediately following heating period.

The V_(BASE) _(—) _(POS) and V_(BASE) _(—) _(NEG) voltage measurements are taken by the MCU when the triac is off to evaluate the proper operation of the positive and negative halves of the triac, and also to remove the power supply ripple effect on the temperature measurements. If either V_(BASE) _(—) _(POS) and V_(BASE) _(—) _(NEG) voltage departs from a known DC offset for more than allowed preset value, the MCU detects a triac Q2 failure, and activates the crowbar circuit Q1 R2 to blow the fuse F2 and disconnect the heating element from the power. This arrangement provides protection not only from a short in any half of the triac, but also in case of excessive leakage current through the triac.

The same circuit arrangement can be used with a single circuit heating element comprising PTC wire. In this case the negative half cycle resistance reading may be used further to improve accuracy of the measurement and to assess the condition of the negative part of the AC switch (triac).

The same circuit arrangement can be used with a multiple circuit heating element comprising PTC wire, where the heating element circuits are sequentially connected (one at a time) to the control circuit for resistance measurements by an additional switching circuit. In this case the control circuit may provide condition evaluation for all AC switches in the heating element operation circuitry.

For the non-PTC types of the AC powered heating elements, or when other techniques are used to control the heating element temperature, the circuits of FIG. 2-FIG. 8, FIG. 9, and FIG. 11 A, and FIG. 11 B provide positive detection of the AC switch failure for both positive and negative halves of the switching structure.

When no crowbar circuit is available, and/or no logic driven protection circuitry is used, a Duty Cycle Protection method provides adequate protection from a short circuit failure of either part of the AC switching element.

Duty Cycle Protection is a passive technique that provides adequate electronic AC switch failure protection without use of extra active components or sophisticated algorithms. The hardware arrangement is given on FIG. 14. The principle of operation is illustrated by diagrams of FIG. 15. Under normal working conditions the triac Q1 is triggered every third cycle of the power line voltage. The heating element HT1 is constructed to provide adequate power at a 33.3% duty cycle. The Fuse F1 is sized to allow continuous operation at an average current corresponding to the 33.3% duty cycle, and blow, when the current is exceeded by more than 15%. If either positive or negative half of the triac fails due to a short, the average current doubles as the duty cycle goes 66.7%, and the fuse F1 blows. If both halves of the triac fail due to a short, the average current triples, and the fuse F1 blows even faster.

The bi-polar temperature control and/or AC switch protection mechanisms described above can be used on any AC powered devices, where a complete or partial failure of the AC switch may lead to a dangerous situation or undesired performance. Examples of such devices include but are not limited to motor controls, light controls, microwave ovens, conventional ovens, etc.

The AC switch may comprise a triac, a triac equivalent, a MOSFET AC switch, a thyristor based AC switch, a solid state relay, or any other electronic circuit capable of controlling AC power in response to the control signal.

It should be noted that in the above described circuits, an inductor or a capacitor may be used in place of the resistor to measure current. As one skilled in the art would know, differing sources of impedance would suffice.

It is also understood that other arrangements of the voltage and current based AC switch failure detection circuits may be employed by those skilled in art to provide detection of the positive and negative halves of the AC switch without departing from the scope and spirit of the present invention.

The embodiments herein described are provided for the purpose of illustration and not limitation of the present invention. 

What is claimed is:
 1. A safety circuit for use in disrupting power to a heating element of an appliance to be powered through a solid state switch from an AC power source having a positive half cycle and a negative half cycle delivering power, including detection circuitry for detecting a low resistance condition in said solid state switch during the positive half cycle of the AC power through said solid state switch and detecting a low resistance condition in said solid state switch during the negative half cycle of the AC power through said solid state switch to generate a fault signal to interrupt power to said heating element whenever said low resistance condition is detected while said solid state switch is not activated.
 2. A safety circuit as set forth in claim 1, said detection circuitry comprising an NPN transistor switch responsive to the positive voltage drop across said solid state switch and a PNP transistor switch responsive to the negative voltage drop across said solid state switch, said PNP transistor switch connected to the base of said NPN transistor switch to actuate said NPN transistor in response to either positive or negative voltage drop across said solid state switch, said detection circuitry providing a high logical level output to a microcontroller whenever said solid state switch is conducting, said microcontroller being arranged to generate said fault signal to interrupt power to said heating element if said high logical level is received while said solid state switch is not actuated.
 3. A safety circuit as set forth in claim 2, said solid state switch being a triac.
 4. A safety circuit as set forth in claim 2, said microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.
 5. A safety circuit as set forth in claim 1, said detection circuitry detecting the level of current through solid state switch during either half cycle of the AC power to generate said fault signal to interrupt power to said heating element when said current through said solid state switch reaches a predetermined level.
 6. A safety circuit as set forth in claim 5, said solid state switch being a triac.
 7. A safety circuit as set forth in claim 5, including a microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.
 8. A safety circuit as set forth in claim 5, said detection circuit including a current transformer.
 9. A safety circuit as set forth in claim 8, said solid state switch being a triac.
 10. A safety circuit as set forth in claim 8, including a microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.
 11. A safety circuit as set forth in claim 5, said detection circuitry including an optocoupler connected across a voltage divider for detecting the level of current through solid state switch during either half cycle of the AC power to generate a fault signal to interrupt power to said heating element when said current through said solid state switch reaches a predetermined level.
 12. A safety circuit as set forth in claim 11, said solid state switch being a triac.
 13. A safety circuit as set forth in claim 11, including a microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.
 14. A safety circuit as set forth in claim 5, said detection circuitry including two NPN transistors connected to a load resistor in series with the solid state switch for detecting the level of current through solid state switch during either half cycle of the AC power to generate said fault signal to interrupt power to said heating element when said current through said solid state switch reaches a predetermined level.
 15. A safety circuit as set forth in claim 14, said solid state switch being a triac.
 16. A safety circuit as set forth in claim 14, including a microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.
 17. A safety circuit as set forth in claim 5, said detection circuitry including inversely parallel diodes connected in series with said solid state switch for detecting the level of current through solid state switch during either half cycle of the AC power to generate said fault signal to interrupt power to said heating element when said current through said solid state switch reaches a predetermined level.
 18. A safety circuit as set forth in claim 17, said solid state switch being a triac.
 19. A safety circuit as set forth in claim 17, including a microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.
 20. A safety circuit as set forth in claim 1, said detection circuitry including a voltage divider connected across said solid state switch through a high resistance to generate an analog signal detecting the voltage level of solid state switch during either half cycle of the AC power to generate said fault signal to interrupt power to said heating element when said voltage falls to a predetermined level, and a converter for said analog signal to deliver a signal to a microcontroller unit to generate said fault signal.
 21. A safety circuit as set forth in claim 20, said solid state switch being a triac.
 22. A safety circuit as set forth in claim 20, said microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.
 23. A safety circuit as set forth in claim 1, said detection circuitry including a current divider connected to said solid state switch to generate an analog signal detecting the current level of solid state switch during either half cycle of the AC power to generate said fault signal to interrupt power to said heating element when said current reaches a predetermined level, and a converter for said analog signal to deliver a signal to a microcontroller unit to generate said fault signal.
 24. A safety circuit as set forth in claim 23, said solid state switch being a triac.
 25. A safety circuit as set forth in claim 23, said microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.
 26. A safety circuit for use in disrupting power to a heating element of an appliance to be powered through a solid state switch from an AC power source having a positive half cycle and a negative half cycle delivering power, said solid state switch being triggered to supply power to said heating element during only a predetermined number of cycles of said AC power source, and a circuit interrupter in series with said heating element and solid state switch to disrupt power to said heating element when a larger current is established indicating said solid state switch is passing more than said predetermined number of cycles to said heating element.
 27. A safety circuit as set forth in claim 26, said circuit interrupter being a fuse.
 28. A safety circuit for use in disrupting power to dual heating elements each formed of a positive temperature coefficient flexible wire and powered through a solid state switch by a respective cycle of the AC power source having a positive half cycle and a negative half cycle delivering power, including circuitry sensing the voltage across said solid state switch through each cycle of the AC power source, and a micrcontroller comparing said voltage to a preset value to determine if either of said heating elements reaches a predetermined level to generate a fault signal to interrupt power to the heating element having reached the predetermined level.
 29. A method for controlling temperature of an AC operated multiple circuit heating element, whereas at least one circuit is operated on a positive half cycle of the AC power line, and at least one circuit is operated on a negative half cycle of the power line, the circuits being operatively connected to the power line by at least one solid state AC switch, having a control terminal capable of turning said AC switch on for a duration of a half cycle of an AC power line, comprising: inserting an impedance in series with the said at least one AC switch, measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a positive half cycle of the AC line, while said AC switch is not actuated, and registering the received value as a positive bias voltage, measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a negative half cycle of the AC line, while said AC switch is not actuated, and registering the received value as a negative bias voltage, measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a positive half cycle of the AC line, while said AC switch is actuated, and registering the received value as a positive active voltage, measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a negative half cycle of the AC line, while said AC switch is actuated, and registering the received value as a negative active voltage, measuring AC line power voltage magnitude, and registering the received value as a line voltage, calculating resistance of said at least one circuit of said heating element operated on a positive half cycle of the AC power line by taking a ratio of the line voltage to a result of subtraction of said positive bias voltage from said positive active voltage, calculating resistance of said at least one circuit of said heating element operated on a negative half cycle of the AC power line by taking a ratio of the line voltage to a result of subtraction of said positive active voltage from said positive bias voltage, supplying power to said at least one circuit of said heating element operated on a positive half cycle of the AC line if the calculated resistance value of said circuit is below a predefined value, and supplying power to said at least one circuit of said heating element operated on a negative half cycle of the AC line if the calculated resistance value of said circuit is below a predefined value.
 30. A method for protecting of an AC operated appliance, whereas a load is operatively connected to the power line by at least one solid state AC switch having a control terminal capable of turning said AC switch on for a duration of a half cycle of an AC power line, said appliance further having a disconnect means for operatively disrupting power to the load, comprising the following: inserting an impedance in series with the said at least one AC switch, measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a positive half cycle of the AC line, while said AC switch is not actuated, and registering the received value as a positive bias voltage, measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a negative half cycle of the AC line, while said AC switch is not actuated, and registering the received value as a negative bias voltage, actuating said disconnect means when an absolute value of either the difference between said positive bias voltage and a preset positive bias value, or the difference between said negative bias voltage and a preset negative bias value, exceeds a predefined value.
 31. A method as set forth in claim 30, whereas said preset positive bias value and said preset negative bias value are substantially equal. 